Numerous types of proportional pneumatic flow control valves are presently available which accurately control flow over a wide range of flow control applications. However, a problem with current flow control valves is that they generally do not offer precision flow at lower rates. For balanced valves the valve percentage of error for the full scale of the valve may be too great; and, the low flow imprecision can be too problematic to the application. As such, to control such a system with PID controllers to eliminate such wide swings that are inherent within the system remains very difficult.
Unbalanced valves are (usually) simpler designed, smaller valves with low pressure drops across the valve. Advantages include a simpler design with fewer potential leak paths at the seat and a lower capital cost. Disadvantages include a limited sizes, since with a large unbalanced valve the forces needed to seat and hold the flow often becomes impractical. A general solution is to maintain holes through the plug in order to achieve an easier shut off as the plug does not have to overcome static forces. However, such a solution creates an additional leak path and at a cost that is generally higher.
In trim control applications, the selection of balanced or unbalanced designs is not entirely straightforward, and will ultimate be based upon the selection and design of the actuator. The actuator design is based on the thrust force which is required to open and close the valve, with the returning forced being supplied by either a spring or diaphragm. Selection of an actuator and spring range must be capable of handling that thrust force which is available in the selected size of valve.
To date the only conventional way to proportionally control the flow rate of a medium with flow control valves over wide swings in flow and pressure is to use two separate mechanical valves in sequence. Using two independent valves in parallel or in series will increase accuracy over particular segments of the full flow range. One valve is typically used for lower flow rates, and then a larger valve subsequently compensates for the higher flow rates. Both valves are independently controlled.
Consequently, there is a need for a single balanced valve mechanism that allows for an increase in the precision of low flow output during the initial flow performance of a device's full-scale flow capability while also allowing for higher volume flow for a remaining portion of the full-scale output.